Various types of dispensing devices are used to discharge liquids having different characteristics, such as temperature and viscosity. Such devices include both contact devices, in which a portion of the dispensing nozzle contacts the substrate, and non-contact devices, in which the nozzles are spaced from the substrate. Contact devices typically dispense beads or surface coatings of liquids such as heated thermoplastic liquids. Non-contact dispensing devices may dispense, for example, either strands or droplets of liquid.
When dispensing continuous strands of high viscosity viscoelastic nonconductive material such as hot melt adhesive, it is often desirable to introduce specific movement of the strand while it is in the air prior to its contact with the substrate so that it forms a desired pattern on the substrate. This allows the single strand of adhesive to spread over a wide area of the substrate and thereby achieve higher strength adhesive bonds and faster production speed. One manner of deflecting a dispensed continuous strand of hot melt adhesive involves the use of pressurized air jets. More specifically, certain dispensing nozzles incorporate air orifices disposed about a central liquid discharge orifice. The air jets discharged from these air orifices impact the continuous strand of adhesive upon its discharge from the central orifice and thereby impart a pattern, such as a swirled pattern, to the strand of adhesive. The swirled continuous strand of adhesive material then contacts the substrate forming a swirled pattern.
While the use of so-called swirl nozzles has adequately addressed the needs of many different applications, improvements related to precisely deflecting a dispensed continuous strand of highly viscous viscoelastic nonconductive liquid are necessary for various reasons. For example, proper placement of a continuous strand of such a liquid can be difficult when a substrate is irregularly shaped or, for example, in cases where the dispenser must place an adhesive bead close to an edge of the substrate. Also, orientation of the substrate relative to the dispenser can make accurate placement of the continuous strand difficult. For example, in certain applications the substrate may not be located directly below the dispenser.
Certain dispensing apparatus in the past have utilized one or more electric fields to affect the flight of dispensed, minute droplets of low viscosity liquid. This liquid may be atomized into a fine particle spray for providing a uniform coating on a substrate. This type of application, however, does not address the unique problems and issues involved when dispensing continuous strands of high viscosity viscoelastic, nonconductive liquids, such as hot melt adhesives. In such cases, it is not feasible to atomize the liquid and, in fact, at normal dispensing pressures of about 1500 psi and below, it is not possible to atomize such liquids. Moreover, the nonconductive nature of these liquids and the fact that they are not corona chargeable introduces problems associated with imparting a sufficient charge throughout the liquid.
It would therefore be desirable to provide dispensing system capable of dispensing a continuous strand of highly viscous viscoelastic nonconductive liquid, and deflecting the dispensed continuous strand in a precisely controllable manner. This may include introducing a desired pattern, such as a swirled pattern, to the continuous strand or deflecting the continuous strand to follow a desired path using one or more electric fields.